Irrigation Scheduling for Spring Wheat in Southern Alberta

Irrigation management is about controlling the rate,amount, and timing of applied irrigation water in a planned and efficient manner. With good irrigation management, a spring wheat crop can have high yieldand quality potential.

Irrigation Management

The goal of irrigation management is to use available irrigation water effectively in managing and controlling the soil moisture environment of crops to do three things: promote the desired crop response, minimize soil degradation, and protect water quality.

Proper irrigation management requires a good understanding of a number of factors:

soil fertility (crop nutritional requirements)

soil-water-plant relationships

crop type

crop sensitivity to water stress

crop growth stages

availability of a water supply

climatic factors that affect crop water use such as rainfall, temperature, humidity, and net radiation

irrigation system capabilities and limitations

Equipped with such knowledge, an irrigator can develop a workable and efficient irrigation scheduling program.

Strategies

A workable and efficient irrigation management strategy should be crop-specific. Crop-specific irrigation management strategies mean available water is used efficiently to meet a specific crop’s water requirements for maximum water productivity.

Generally, the goal is to ensure that water is available at germination and in early development by applying light, frequent irrigations (if there is no rainfall). This method promotes vigorous growth and replenishes and increases available soil water content in the entire root zone during the pre-flowering growth stages. Such a strategy will allow modern sprinkler irrigation systems to keep up to crop demand during the peak water use period, which typically occurs during the flowering and fruit-formation growth stages.

Crop-specific irrigation management strategies are usually applied to adjust for the following differences among crops:

effective root zones

sensitivity to water stress

types (cool versus warm-season)

vulnerability to diseases at various crop growth stages

response to soil fertility levels

plant population/densities

physiologic maturity (timing of last irrigation)

potential income

Spring Wheat Water Needs

Spring wheat uses water for growth and cooling purposes. The water requirement or evapotranspiration (ET) for spring wheat depends on variety, growth stage, canopy density, climatic conditions, and irrigation and crop management.

Spring wheat grown under optimal conditions (well-fertilized, well-irrigated, seeded in standing stubble, pest-free, and uniform and optimum canopy) requires420 to 480 mm of water per growing season in southern Alberta. When seeded into soil with available water between 60 and 100 per cent, spring wheat will germinate, grow rapidly, and reach a peak water use of nearly 7 mm per day during the flowering and fruit-formation growth stages (Figure 1).

Typically, the roots of spring wheat grow to an effective water extraction depth of 100 cm in a well-developed soil. Root distribution is concentrated near the surface; hence, spring wheat obtains more than 70 per cent of its seasonal water from the upper 50 cm of the active root zone. The active root zone changes from a few millimetres at emergence to a maximum depth of 100 cm at the flowering growth stage.

Figure 1. Daily water use during different growth stages of irrigated spring wheat in southern Alberta. Shaded area indicates variation in spring wheat water use depending on plant type, cultivar, and climatic conditions.

Irrigation Scheduling Strategy

Effective spring wheat irrigation scheduling uses soil water levels in the root zone as a measure for starting and stopping irrigations. Adequate soil water is critical for spring wheat during the emergence, vegetative(pre-flowering), flowering, and fruit-formation growth stages. Ideally, soil water content in the 0 to 50-cm depth should be greater than 60 per cent of available at planting.

Spring wheat needs to have water for germination and root development during the early stages of growth. If seeded in a dry seedbed (less than 60 per cent of available in the 0 to 50-cm depth) in late April before irrigation water is available, the first and subsequent irrigations(15 mm per irrigation event) should be applied as soon as irrigation water is available in early May. These irrigations should be light and frequent to maintain a moist soil surface, prevent crusting, and encourage rapid emergence and early root development.

If well-fertilized, a pest-free spring wheat stand will reach maximum grain yield and quality if ample water is available in the root zone during the tillering and flowering growth stages. To ensure that ample water is available to spring wheat during the vegetative growth stages (i.e. tillering to late boot), available soil moisture should not be depleted to less than 60 per cent in the upper 50 cm of the 100-cm root zone.

Any irrigation applied during the vegetative growthstages should start when the available soil water is near65 per cent of available to prevent the available soil water from being depleted to less than 60 per cent.

Maintaining available soil water above 60 per cent in the upper 50 cm depth during the vegetative growth stages translates to light and frequent irrigation applications. Irrigation water applied during the vegetative growth stages should meet crop water requirements and build up soil water to near field capacity in the 50 to 100-cm zone for later crop use during the peak water use period when flowering is occurring.

In general, spring wheat is most sensitive to inadequate soil water during the flowering growth stage. Inadequate soil water during this stage results in flower abortion.

Spring wheat roots reach maximum extension at the flowering growth stage. To ensure that soil water is adequate throughout the root zone during flowering, the monitoring depth of the root zone should be increased from 50 cm to 100 cm at the early heading growth stage, and soil water should not be depleted to less than60 per cent of available (i.e. allowable depletion should not be greater than 40 per cent of available).

Irrigations should be scheduled to fill the entire root zone (100 cm) to field capacity at the late boot- early heading growth stages to avoid applying irrigations during the flowering growth stage when the crop is most vulnerable to Fusarium Head Blight (FHB).

Increasing the irrigation management root zone from50 cm to 100 cm at the late boot- early heading growth stages requires less frequent and larger irrigation volumes and results in increased water availability to the mature spring wheat roots. This increased time between irrigations keeps the canopy dry, discouraging the growth of FHB.

Fusarium Head Blight

The strategy of scheduling irrigations to fill the entire root zone (100 cm) to field capacity at the beginning of the late boot to early heading growth stages, combined with the application of appropriate fungicides, may help reduce the severity of FHB infection. The risk of FHB increases when the canopy is moist due to rainfall or irrigation during flowering.

Applying irrigation to recharge the 100-cm root zone to field capacity at the start of heading should be done whether or not the allowable depletion soil water content (irrigation trigger) has been reached. The next irrigation event should be applied after flowering is complete (about 10 days after the first flower appearance) and when soil water content is near 60 per cent of available in the 100-cm root zone (Table 1).

As an additional management tool for managing the risk of FHB, it is recommended that spring wheat growers consider increasing seeding rates, which helps reduce tiller formation and shorten the flowering period for the entire crop, thus reducing the time that irrigation should be avoided.

Final Irrigation

The timing of the last irrigation to refill the root zone for spring wheat depends largely on the soil texture. The final irrigation may be applied at the soft dough growth stage when spring wheat is grown in most soils except for loamy sand soils, which are limited by the lower water-holding capacity. The last irrigation to refill the root zone may be needed between the soft dough and hard dough stages on loamy sand soils.

About 80 mm of water is required to carry spring wheat from soft dough to physiologic maturity in southern Alberta. No irrigation water is needed once the heads have completely turned colour from green to brown since the crop is mature at this point and yields have been established.

Table 1. Soil texture-based estimation of total available water and water amounts per irrigation event for spring wheat during the spring wheat growing season

Water required to replenish soil to field capacityat 40% allowabledepletion (mm)

Total available water (mm)

Water required to replenish soil to field capacityat 40% allowabledepletion (mm)

Loamy sand

57

23

114

46

Sandy loam

70

28

140

56

Loam

90

36

180

72

Sandy clay loam

76

30

152

61

Silt loam

100

40

200

80

Clay loam

100

40

200

80

Silty clay loam

110

44

220

88

Sandy clay

86

35

172

69

Silty clay

106

43

212

85

Clay

96

39

192

77

Soil texture

The irrigation amounts required to replenish the root zone once allowable depletion soil water level is reached will vary with soil texture and growth stage (Table 1).

Conclusion

Using suitable irrigation strategies with spring wheat can mean a healthy crop with high yield and quality potential. In addition to ensuring that the spring wheat crop is well-fertilized and well-protected from pests, growers are encouraged to properly manage irrigation by regularly monitoring soil water to ensure that the availability of water does not become a limiting factor in producing a high-yielding spring wheat crop.
Applying irrigation just before the available soil water is depleted to 60 per cent and replenishing available soil water near field capacity in the appropriate root zones will greatly assist in producing a high-quality and high-yielding spring wheat crop.